This disclosure relates generally to x-ray generation systems. In particular, this disclosure relates to systems and methods for protection of bias circuits from high voltage transients and discharge/spit protection in x-ray tubes.
An x-ray tube generally includes a cathode assembly and an anode assembly disposed within a vacuum vessel. The anode assembly includes an anode having a target track or impact zone that is generally fabricated from a refractory metal with a high atomic number, such as tungsten or a tungsten alloy. The anode is commonly a rotating disk. The cathode assembly is positioned at some distance from the anode assembly creating a vacuum gap between the cathode assembly and the anode assembly, and a high voltage potential difference is maintained therebetween. The cathode assembly emits electrons in the form of an electron beam that are accelerated across the potential difference and impact the target track at a focal spot of the anode at a high velocity. As the electrons impact the target track, the kinetic energy of the electrons is converted to high-energy electromagnetic radiation, or x-rays. The x-rays are then transmitted through an object such as the body of a patient and are intercepted by a detector that forms an image of the objects internal anatomy.
In an x-ray tube, the focal spot can be controlled and deflected electrostatically through bias voltages. This is accomplished by applying different bias voltages at a number of electrodes within the cathode assembly. The cathode assembly generally includes at least two pairs of electrodes positioned on opposite sides of the cathode filament to control the size and deflection of the electron beam. A bias voltage is independently applied to each of the electrodes to focus and/or deflect the electron beam. In an x-ray tube with focal spot wobbling, the focal spot is wobbled electrostatically between two positions on a target track of an anode during a scan sequence. Electrically isolating the cathode septum and applying a continuously varying bias voltage to the cathode filament provides two unique focal spots that can be controlled with bias voltages. It is generally preferable to minimize the bias voltages at the electrodes to reduce the risk of insulation breakdown and improve reliability of the x-ray tube.
One of the potential problems in an x-ray tube is that there are considerable high voltage transients induced across the bias circuits, resulting in possible damage of the high voltage cable assembly and certain components within the high voltage generator, when a spit (either a vacuum discharge or a vacuum arc) occurs. A typical high voltage transient within the bias circuits could be as high as several tens of kilovolts for an x-ray tube. This presents a serious reliability problem, as the basic insulation level for the minor insulation along the bias circuits is not high enough to withstand these high voltage transients.
Therefore, there is a need for a system and method that prevents the occurrence of high voltage transients within an x-ray tube, and in particular provides protection of bias circuits from high voltage transients and spit protection in x-ray tubes.